Java Callback Simplification Proposal

[Jim Marino <jmarino@bea.com>]

All,

Bellow is a proposal from BEA to simplify callback mechanisms in the  
Java C&I specifications. I will work on the appropriate steps of  
getting this into a JIRA so that it can be tracked.

Thanks,
Jim

This proposal aims to simplify using asynchronous communication in   
the SCA Java programming model by:

1. Eliminating callback objects.
    - Removing ServiceReference.get/setCallback()

2. Removing the ability for service providers to get callback IDs.
    - Moving CallableReference getCallbackID() to ServiceReference)

3. Saying callback IDs are irrelevant in conversations.
    - Making ServiceReference.setCallbackID(Object id) illegal and
      ServiceReference.getCallbackID() (assuming #2) return null for
      conversational interactions.

4. Removing the distinction between stateless and conversational
    callback interfaces.
     - Removing the ability to specify conversational on callback
       Interfaces.

By simplification, we mean as a primarily reducing the number of   
concepts and APIs a developer must understand to program  
asynchronous  services. This will entail some reduction in  
capability. However,  this reduction, we argue, affects edge-cases  
that can be accommodated through alternative techniques. A secondary  
aspect of simplification concerns implementation. By removing the  
requirement for runtimes to provide infrastructure for  handling edge  
cases that can be accommodated through relatively simple application  
programming techniques,implementing callback capabilities becomes  
fairly straightforward.

[RATIONAL]

The rational for proposed items is dealt with individually in this  
section.

1. ServiceReference.setCallback(Object object) and  
ServiceReference.getCallback()
------------------------------------------------------------------------ 
---------------------------------------------

This API exists as a mechanism to route callbacks to instances other  
than an instance of the client component making a forward invocation.  
This provides two capabilities.  Clients do not have to implement a  
callback interface. The API also allows clients to instruct the  
runtime to perform callback dispatching for stateless interactions  
that execute concurrently. A common example of this is inventory  
check. A client component needs to perform an inventory check for  
multiple line items. As an efficiency, it written to invoke the same
inventory service using a non-blocking call multiple times. For each  
invocation, it wishes to dispatch the associated callback to a  
different handler instance it creates.  For example:

ServiceReference<InventoryService> serviceReference = ..

for (LineItem item: purchaseOrder.getLineItems()) {
     CallbackService handler = new CallbackServiceImpl();
     serviceReference.setCallback(handler)
     serviceReference.getService().checkInventory(item);
}

This can be done using the CallableReference.getCallbackID():

// code that conducts the forward invoke:

ServiceReference<InventoryService> serviceReference = ..

for (LineItem item: purchaseOrder.getLineItems()) {
     String id = // generate or assign the callback ID
     // store the callback id and whatever information the callback  
needs
     cache.put(id, handler);
     serviceReference.getService().checkInventory(item);
}

Callback processing can either be done directly by a component  
implementation instance or delegated to CallbackServiceImpl as shown  
below:

// code in the same class that handles the callback:
public void onCallback(InventoryStatus status) {
     String id = requestContext.getServiceReference().getCallbackID();
     CallbackInfo info = .. // retrieve the stored callback
information by using the callback id
     CallbackService handler = new CallbackServiceImpl(info);
     handler.onCallback(status);
}

Some advantages to eliminating get/set/Callback() are:

a. Reduces the number of concepts and APIs a developer will be  
presented with without affecting common functionality. In other  
words, this API is primarily used for edge-cases and removing it  
simplifies the common case.

b. Application migrations and versioning are easier using the   
alternative approach. By using an object instance, runtime  
implementations would be responsible for handling class versioning
for in-process interactions. By avoiding serialization and the use of  
a specific classes, and instead storing callback data,  
CallbackServiceImpl can be versioned and substituted for callbacks  
that have not yet occurred.

c. Runtime performance characteristics are likely to improve. For  
cases where the client is stateless, routing to the specific instance  
set through setCallback() requires that the callback be routed to the  
specific JVM where the instance is hosted. Eliminating routing to a  
particular instance allows the callback to potentially be sent to any  
JVM where the client component is hosted.

d. Eliminates the need for runtime implementations to manage garbage  
collection of callback objects. For example, if a callback is never  
made, the runtime at some point will need to remove the object from  
storage or memory.


2. Moving CallableReference.getCallbackID() to ServiceReference
------------------------------------------------------------------------ 
--------------------

We propose removing getCallbackID() from access by service providers  
as it is cannot be used for correlation in third-party components  
except when the third-party is configured with a callback to the same  
client component. For example in the stateless case:

A---------B--------C

The only way C can callback to A is either by calling back B or  
having B pass a CallableReference to C.  The callback ID is not  
sufficient, in itself, to be used by arbitrary code to send a message  
that qualifies as a callback in this bidirectional exchange.  For  
example, if B has clients other than A, the callback ID doesn’t have  
the information to tell which client it is for.


Moving CallableReference.getCallbackID() to ServiceReference   
simplifies the Java programming model by:

a. Eliminating an API that can only be used in very particular  
circumstances.
b. Removing the need for a runtime implementation to create and  
maintain a distributed domain-wide mapping of callback ids to  
callback endpoints. Note that routing information cannot be encoded  
in the callback id as it can be set by the application.


3. Making ServiceReference.setCallbackID(Object id) illegal and  
ServiceReference.getCallbackID() (assuming #2) return null for  
conversational interactions.
------------------------------------------------------------------------ 
------------------------------------------------------------------------ 
----------------------------------------------------------------------
Having both callback ids and conversation ids for bi-directional  
conversational wires is confusing and not needed as the conversation  
id will be used for correlation. Sometimes a client may wish to  
correlate callback invocations using the callback id. For  
conversational callbacks, correlation using the callback id is  
limited. Since conversations are serial between a particular client  
instance and a provider instance, the additional correlation that may  
be needed is between different invocations to the provider within the  
same conversation. In other words, conversations by nature correlate  
invocations between a client and provider instance. If finer-grained  
correlation is needed such as between multiple invocations in the  
same conversation, correlation data can be included as part of the  
service operation data (e.g. “line item number” can be passed as a  
parameter in the example above).

This simplifies the Java programming model by clarifying the  
relationship between callback and conversation ids.


4. Removing the ability to specify stateless or conversational  
callback service contracts
------------------------------------------------------------------------ 
-----------------------------------------------------
We propose wires be either conversational or stateless in both  
directions and that it be specified as part of the forward service  
contract. In other words, @Conversational cannot be specified on the  
callback interface definition.  We don't believe having a stateless  
callback for a stateful forward invocation and vice versa makes much  
sense.